The present invention relates to a method for producing a tin-plated film in which whiskers are hard to be generated and hard to grow, and a tin-plated film produced by using the same method.
Tin plating has been generally employed as a metallic surface processing method for ornaments, electronic components and structural parts because tin plating is inexpensive in costs, and brings about excellent durability, satisfactory appearance and an excellent soldering property.
However, it has been known that needle crystal consisting of single crystal of tin called “whiskers” are generated on a tin-plated surface. The whiskers generally are 1 through 2 μm diameter, and several micrometers through several millimeters long, which is easy to be generated particularly on a brightened tin-plated film to which a brightening agent is added. If whiskers are generated and grows in electronic components, it becomes a cause of short-circuiting in circuits or between terminals and noise generation. In particular, since recent electronic components are inclined to downsizing, high concentration, and feeble currents, generation of whiskers is apt to arise a failure or obstruction.
Therefore, in order to secure reliability of electronic components to which tin plating is applied, various methods for preventing whiskers from being generated have been developed.
For example, JP-A-55-138967 describes a “method for heating a tin-plated product in a non-oxidizing atmosphere to a temperature exceeding the melting point of tin.”
JP-A-57-126992 describes a “method for quickly heating to a predetermined temperature between 180° C. and the melting point of tin at a programming rate of 5 through 100° C. per second after tin plating, and holding at the predetermined temperature within 180 seconds.”
JP-A-57-145353 describes a “method for heat-treating at 70° C. through 100° C. within five hours after tin plating.”
JP-A-5-33187 describes a “method for plating tin to a thickness of 0.15 μm or more, subsequently making the entire tin layer into a Cu—Sn diffusion layer with a copper base material by heat treatment, and plating tin thereon to a pure tin thickness of 0.15 through 0.8 μm.”
However, the prior art technologies described above have the following problems and shortcomings.
In the technology described in JP-A-55-138967, since a substrate to be plated is heated in a non-oxidizing atmosphere, it is possible to prevent the surface of the substrate from being oxidized and discoloring. However, a facility for adjusting the atmosphere is required, and at the same time the running costs are increased. Therefore, there is a problem in that productivity is not satisfactory.
In the technology described in JP-A-57-126992, an effect of preventing whiskers from being generated on a tin-plated film can be obtained. However, if quick heating is carried out at a programming rate of 5 through 100° C. per second, there arises a problem in that the surface of a substrate not covered with a tin-plated film is oxidized and is discolored.
In the technology described in JP-A-57-145353, an effect of preventing whiskers from being generated on a tin-plated film is obtained by heating for a long period of time. However, there arises a problem in that the surface of a substrate to be plated, which is not covered with a tin-plated film, is discolored, and productivity is not satisfactory because the heating time is long.
In the technology described in JP-A-5-33187, there is a problem in that the processes are complicated and productivity is not satisfactory. Also, there is another problem in that the surface of a substrate to be plated, which is not covered with a tin-plated film, is discolored.
In addition, as described in JP-A-10-96095 and “Whisker Evaluation of Tin-plated Logic Component Leads” D. Romm and D. Abbott, (USA), Texas Instruments Application report SZZA037A, pages 7 through 10, February Issue, 2003, there is a method for making grain sizes of tin coarse, for example, 2 μm to 8 μm. However, whiskers cannot be effectively prevented.
Further, as disclosed in “X-Ray Diffraction and Whisker growth—Is there any correlation?” M. Jordan and G. Strube, (German) International Conference on Lead Free Electronics, Jun. 12, 2003, there is a method for controlling the priority orientation plane of tin crystal. However, whiskers cannot be effectively prevented as well.